One well known form of an image sensor includes a PIN photodiode and four transistors for each pixel. Known circuits of this kind have limitations. One limitation is that PIN photodiode pixels have a low output voltage swing due to the large capacitance of the floating diffusion onto which the PIN photodiode charge is transferred. The floating diffusion capacitance is difficult to accurately control due to the pixel architecture. The diffusion capacitance depends on the gate oxide, the source diffusion and the routing capacitance.
Another limitation is that the voltage on the floating diffusions must be kept sufficiently high to maintain full depletion of the photodiode. Since, the voltage on the floating diffusion is light signal dependent, this imposes a limit on the swing of the pixel. This in turn requires that the floating diffusion be reset to as high a voltage as possible. However, the reset voltage is limited by amplifier swings and the power supply.